Technology whereby unevennesses are formed on the surface of a tread portion by adding a fine substance to a rubber material is known as a means for increasing the frictional force on ice of a studless tire (for instance, refer to Patent Documents 1 to 3).
With this technology, frictional force on ice is enhanced by unevennesses formed on the surface of a tread portion that drain the water film generated on the ice surface under high temperatures and scratch the surface of the ice under low temperatures. However, this technology is incapable of adequately guaranteeing the frictional force on ice of prior art, and even brings about a reduction in wear resistance. Therefore, further improvement is desired.